Most modern highly refined four cycle diesel engines today, in the displacement range from 10 to 16 liters, run at about 2000 rpm with average piston speeds of about 2000 feet per minute. Most of the engines are used for over-the-highway longdistance hauling (class 8 trucks) with power outputs up to 400 to 450 BHP. At least in the United States, the engines utilize four-valve-per-cylinder designs. Some of these engines are also used for industrial, marine and construction equipment. Other than some much larger military industrial, generating and marine engines which have used four valves per cylinders practically since the end of World War II, these newer American engines are the only four-valve-per-cylinder large diesels in the world today, and are produced in relatively small numbers by only about five U.S. companies. In the rest of the industrial world (Europe and Japan) as well as in the U.S. for all other applications (class 7 and smaller trucks, agricultural, marine, industrial, buses, etc.) the bulk of production is based in old two valve technology. The situation is mostly due to the multiplicity of manufacturers (dozens) and the myriad of uses (as many as three to four thousand different models being available), as well as the very high costs of development, tooling and manufacturing new designs for such stratified markets.
In spite of these very real difficulties, great progress has been accomplished over the last 25 years, with the advent of the above-mentioned four valve engines, more modern combustion systems, turbochargers and lately, greatly improved electronically-controlled fuel injection systems. These newer engines deliver very high specific outputs and operate with better fuel consumption, lower visible and gaseous emissions and lower noise than their predecessors. These engines have increased reliability and durability as well as reduced oil consumption and extended periods between oil changes.
About 25 to 30 years ago, "swirl" (circular motion of the air charge rotating about the cylinder axis) was introduced, resulting in marked combustion improvements due to increased combustion speed. For the two valve engines of the time it was initially achieved by the use of valve "shrouds" (circular sections of metal welded to the underhead of the intake valves, with such valves prevented from rotation to insure the constant movement of air in the desired direction). Later, the intake ports were developed into a helical shape to achieve the rotational air motion of the air exiting the intake valves, said rotation continuing into the cylinder during the intake stroke as well as the exhaust stroke and well through the combustion and expansion portions of the cycle. All of the above two valve engines were of overhead valve construction design, with actuation by a single in-block camshaft through pushrods and rocker arms, with the valves in-line (the valve axis in a plane parallel to the crankshaft).
At about this time, some of the above-mentioned engines were converted to operate with four valves. The conversion consisted of new cylinder heads with two intake and two exhaust valves, still operated by pushrod and rocker arms but now with both intakes (and both exhausts) in planes transversal to the crankshaft ("across" valves), but with a new member in the form of a "T" bridge, mounted in a post between each pair of valves to allow the single rocker arm to operate both valves at the same time. Although with four valves the geometric air-flow capacity of the engine increased by 30 to 40%, such numbers never materialized for, first, with the "across" valves, the intake valves had to reach all the way across the cylinder head with a very sinuous port and second, because the port further downstream also had helical features to enhance swirl. These two less-than-streamlined approaches significantly reduced the potential increases in air flow, with the final numbers being roughly half of the possible 30 to 40%. The same situation existed with the exhaust ports, except that their aerodynamic characteristics were better for there was no need to induce any air motion with helical features. And so, the final improvements in air flow and swirl were well received and continue essentially unchanged to this day.
The smaller valve size used in four valves design should have resulted in significantly smaller valve stems, springs and retainers. However, no decrease in weight occurred because the "T" bridge itself adds more weight than what is saved on both valves served by each bridge. The dynamic weight of the valve train continues to be as high as it was with the former two valve cylinder heads, and the valve springs are essentially, as stiff, therefore; no improvements were realized on valve train friction and power consumption. With four valves, however, the valve seat bridges, all four of them (as compared to a single bridge for two valve heads) are shorter, stronger and better cooled and, being located roughly halfway from the head center toward the peripheral wall of the bore, are also mechanically less prone to flexing than the single bridge of two valve heads, which, being located nearly on the center of the bore can tend to flex more, contributing to increased valve wear and possible cracking of said bridge. Besides, with two valve heads, for air flow reasons, there is great pressure to make the valves as big as possible, sometimes to the detriment of the bridges or the valves themselves. With four valve technology, and all the increases in air flow provided by larger valve flow areas, the design can be a little more focused towards optimum bridge size, strength and cooling.
Four valve designs also provide for optimum location and position for the fuel injector, nested between all four valves substantially in the center of the head along a vertical axis which is substantially the same or very close to the cylinder axis. The injectors for four valve engines provide a symmetrical injection pattern and a piston bowl concentric to the cylinder diameter, both of which are directionally correct for good mixing and reduced wall-wetting. With the offset, angled injector used with two valve engines, it is very difficult to obtain an even, symmetrical pattern because of complications relating to the hole drillings at the nozzle. While the injector hole geometry may be difficult, it is not impossible to achieve an acceptable compromise even with these compromises for two valve engines. However, it is impossible to center the bowl and the cylinder diameter, resulting in more plume impingement and wall wetting on the bowl walls on the side closer to the injector holes. For these and other reasons, most internationally marketed two valve engines favor a smaller-diameter, deeper bowl than the shallow, large diameter bowl.
Four valve head engines with the smaller valve head sizes, also allow more design flexibility on valve lift, timing and opening duration. It is a general rule that at lifts above 25% of valve head size at least for intake valves, the valve opening area ceases to be the main flow-controlling factor and the port throat area becomes the restrictive flow controller. Although in some cases this rule of nature can be bent, it cannot be broken, for, depending on the streamlining and aerodynamics of the ports sometimes it is possible to use lifts higher than 25% of diameter for intake valves, at least with turbocharged engines. But 25% is a good base for discussion, and it indicates that the smaller valve sizes of four valve heads can also have reduced physical lifts while still providing, both valves combined, greater air flow than a single larger valve with higher lift.
Due to valve train dynamics, a two valve head system, running at the same frequency (i.e. same engine rpm) as a four 15 valve head, requires, at the same forces and accelerations, more time to reach their maximum lift, in other words, more degrees of crankshaft rotation. This mechanical requirement, plus the fact that, in most cases, two valve engines at high speed must make-up through extended valve duration the air flow that they lack from lifted area, generally result in valve opening durations which are longer than is required with the four valve constructions. In reality then, the intake valve opens much earlier than piston TDC (top dead center) and closes well past BDC (bottom dead center), and the exhaust opens earlier in the expansion stroke, well before BDC and closes late in the following intake stroke, well past TDC. The "overlap" around TDC during the scavenging portion of the cycle, when the intake valve opens early and the exhaust closes late is necessary to provide sufficient real time in the gas-exchange process to allow complete evacuation of the spent exhaust gases through the exhaust system as well as to allow the column of intake air, which also has inertia, to start moving early into the cylinder so that maximum valve lift occurs shortly after the point of maximum piston speed to allow the valve to close as soon as thermodynamically and mechanically acceptable after BDC.
Concurrently, and for the same mechanical force and acceleration requirements which force the extended durations of the exhaust period, on most two valve designs the exhaust valve opens earlier than thermodynamically advisable, wasting energy to the exhaust and raising the exhaust temperature (and also the valve temperature) and imposing very large forces on the valve train trying to open the valve against still high cylinder pressures.
By simple geometry, it is easy to see that the late intake closing, well past BDC, results in a reduced effective compression ratio, in some cases much lower than the nominal design ratio. Assuming that, at high speed, through turbocharging, for example, air is still or has just ended entering the combustion chamber due to turbo discharge pressure or the inertia of the air column, as the speed is decreased to the lower ranges, because of lower inertia of the air column and/or lower boost pressure, the intake valve closes later than the point at which air has ceased to enter the cylinder, and the piston is already far past BDC and moving rapidly into the compression stroke, in this case pushing part of the air charge already admitted back into the intake manifold. This condition not only wastes energy twice in admitting then "spitting-back" that part of the air charge, but also reduces the potential volumetric efficiency, which, coupled to the reduced effective compression ratio, results in lower compression pressure and temperature near TDC at the point of injection. The lowered compression temperature results in noisy and inefficient combustion, with high gaseous emissions (particularly NO.sub.x, due to the early combustion and the richer air/fuel ratio), smoke, and particulates.
The longer fuel chemical delay time, the extra fuel injected during such delay, the rapid raise of pressure and temperature of so much fuel essentially igniting all at once, the increased wall wetting during slower speeds provides combustion complications. Couple these conditions to a cold start situation, with the engine cranking at 100 to 150 rpm, and the reasons why some engines start so poorly become very clear: lack of air, low pressure and temperature both from low trapped volumetric efficiency and low effective compression ratio, energy wasted in pumping the air in and "spitting" it back. Four valve designs with lower lift and shorter durations, with valve events closer to TDC and BDC go a long ways towards correcting these inherent problems of two valve designs.
During the last ten years, continued combustion research and improvements in fuel injection systems (higher pressures and electronic timing control, in some cases both beginning and ending of injection), have produced further positive results. Smaller nozzle holes, higher fuel pressures and longer injection duration have been, essentially, shifting the responsibility of mixing the air and fuel from the air motion and energy (swirl) to fuel energy. The smaller injector holes produce less fuel penetration and impingement of the fuel plume against the piston walls (wall wetting), producing a finer mist and keeping the fuel droplets airborne, making it easier to mix with the fuel. Better injection system controls, which have resulted in sharp and clean ends-of-injection tail ends (a notorious cause of visible and gaseous emissions and increased fuel consumption), have allowed an extension of the useful injection period into what formerly was the tail-end of injection. This extended injection duration, the improved mist and shorter fuel plume, the reduced wall-wetting and reduced tail ends have actually lessened the dependency on "swirl" air motion, and by reducing it thus, the air flow has increased. The net results have all been positive to power output, emissions, fuel consumption and noise.
For optimum operating conditions, as well as for maintaining the high compression ratios required (mostly for cold startability) the pistons must come very close to the cylinder head and with extended valve overlap periods, valve cut-outs or pockets must be machined on top of the piston to avoid the valves and pistons from hitting each other. The situation is less critical with four valves, for the pocket depth is less, but then four cut-outs exist rather than two. These cut-outs not only interfere with the air swirl, diminishing it, but during combustion are not really part of the bowl, but "inactive" volumes, particularly as applied to engines using deep bowls and effectively hamper the combustion process, especially during cold start for the air mass favors the colder sections of the chamber, density being an inverse function of temperature. Since the valve pockets or cut-outs are relatively shallow and offer high surface to volume ratios, the air temperature thereabouts is lower than inside the bowl, which negatively affects the density ratios, placing too much air in the inactive areas and not enough in the bowl where it is supposed to be to meet the incoming fuel. Swirl, no matter how good and well-matched it may be to the engine requirements while running full-load at rated speed, is totally inexistent at starting speeds and makes no contribution to mixing or combustion. Finally, and especially during the last decade, great improvements have been made on very modern sparkignition automotive engines featuring four valves for optimum air flow and direct-acting double-overhead camshafts set on a narrow valve included angle (V.I.A.). The engines offer an extremely light and mechanically stiff valve train, with low spring forces and low power consumption, with short valve durations and more judicious valve timing points near TDC and BDC, with each camshaft operating its own bank of intake or exhaust valves for a true cross flow design and with a central source of ignition. And with such engines developing the honest 40% improvement in power offered by their corresponding increases in valve air flow area, while improving effective compression ratios, combustion smoothness and noise and greatly reducing gaseous emissions as well as improving startability.
What is technically needed by future advanced diesel engines are engines featuring the modern trends of smaller sparkignition automotive engines: namely, four valves with no porting compromises for maximum air flow, direct acting, stiff yet lightweight valve trains with low power consumption and improved dynamics to allow the lighter valves to operate faster but with reduced durations for a higher ratio of effective to nominal compression ratios, with a narrow V.I.A. to allow better air flow but mostly to permit the installation of bulkier but more modern fuel injection systems substantially in the center of the head. A piston incorporating special combustion chamber requiring no swirl and combining the necessary valve pockets into active areas of the combustion chamber, such as to allow the maximum possible amount of air to participate in the active combustion process for improved combustion and startability under all conditions, for lower emissions, noise and improved fuel consumption. What is also needed is to provide all of the above-mentioned design benefits and coupling them to a novel injector-hole diffuser approach which provides a wider, less penetrating plume even with increased fuel injection pressures for yet additional benefits in misting, fuel droplet size, mixing and burning with more airborne fuel.